In a nutshell

What is RISC-V?

RISC-V (pronounced "risk-five") is an open source instruction set architecture (ISA) based on established reduced instruction set computing (RISC) principles.

In contrast to most ISAs, RISC-V is freely available for all types of use, permitting anyone to design, manufacture and sell RISC-V chips and software. While not the first open ISA, it is significant because it is designed to be useful in modern computerized devices such as warehouse-scale cloud computers, high-end mobile phones and the smallest embedded systems. Such uses demand that the designers consider both performance and power efficiency. The instruction set also has a substantial body of supporting software, which fixes the usual weakness of new instruction sets.

The project was originated in 2010 by researchers in the Computer Science Division at UC Berkeley, but many contributors are volunteers and industry workers that are unaffiliated with the university.

There are different versions of the instruction set for 32, 64 and 128 bits; operating as little-endian by default.

What is a Debian port?

In short, a port in Debian terminology means to provide the software normally available in the Debian archive (over 20,000 source packages) ready to install and run on systems based in a given computer architecture with the Linux kernel, or kernel-architecture combinations, with other kernels including GNU Mach (from GNU/Hurd) and kFreeBSD (from GNU/kFreeBSD).

What are the goals of this project in particular?

In this project the goal is to have Debian ready to install and run on systems implementing a variant of the RISC-V ISA:

Software-wise, this port will target the Linux kernel

Hardware-wise, the port will target the 64-bit variant, little-endian

This ISA variant is the "default flavour" recommended by the designers, and the one that seems to attract more interest for planned implementations that might become available in the next few years (development boards, possible consumer hardware or servers).

While 32-bit and 128-bit implementations are possible, there are problems with this:

In the context of RISC-V design, they have not been explored as deeply, and tools and resources (e.g. simulators, research cores) as not as well studied and adapted;

For general purpose computers, the focus shifted to 64-bit for many years already, and there isn't a lot of interest in 32-bit architectures except for specific purposes;

32-bit ports in Debian already struggle to compile some large packages of the archive in the last few months/years, a problem that will become worse with time;

ASIC implementations, i.e. "real" CPU chips

SiFive "Freedom U540" SoC (quad-core RV64GC) / "HiFive Unleashed"

At FOSDEM 2018, working production samples of the SiFive "Freedom U540" SoC (quad-core RV64GC) and a corresponding development board ("HiFive Unleashed") have been presented. A limited number of boards from the first production run has been made available in February 2018, a second production run has been done in June 2018.

Planned

In the future further RISC-V-based ASICs are expected, among them a SoC from the lowRISC project, which has described itself as follows:

"lowRISC is a not-for-profit organisation working closely with the University of Cambridge and the open-source community.
lowRISC is creating a fully open-sourced, Linux-capable, RISC-V-based SoC, that can be used either directly or as the basis for a custom design. [...]
Our open-source SoC (System-on-a-Chip) designs will be based on the 64-bit RISC-V instruction set architecture. Volume silicon manufacture is planned as is a low-cost development board. [...]"

FPGA implementations

There are freely available softcores which can be synthesized to an FPGA, such as Rocket, a 64-bit RISC-V in-order core (optionally including an MMU and an IEEE 754-2008-compliant FPU).

Debian port information

Hardware baseline and ABI choice

The Debian port uses RV64GC as the hardware baseline and the lp64d ABI (the default ABI for RV64G systems).

Making the C extension a part of the default hardware baseline for general-purpose binary Linux distributions has been agreed upon between Fedora porters, Debian porters and members of the RISC-V foundation. According to the chairman of the board of the RISC-V foundation, the foundation will provide "a profile for standard RISC-V Unix platforms that will include C as mandatory".

FTBFS, packages that Fail To Build From Source (in riscv64)

Breno Leitão or other people from his team (ppc64el) created the following script, which lists packages that currently fail to build in riscv64, classifying them by number of arches in which it fails (e.g. only in riscv64 or in more than this one) or if there are pending patches or not, etc.; and with links to several related places handy to have (e.g. build logs, BTS).

So it's a nice place to start looking at things that need to be fixed.

Cross compilation

Pre-built toolchains

Since 2018-03-23 both gcc-7-based as well as gcc-8-based cross-toolchains targeting riscv64 are available in unstable. These include glibc and related basic libraries for riscv64 in arch:all packages. As those packages use different library paths than the corresponding "native" (i.e. arch:riscv64) packages and don't include some of the configuration that is part of their "native" counterparts, making full use of them for building packages in a multiarch configuration requires the following steps:

As pbuilder works in a throwaway chroot and deletes it again after it has finished, it is important to bind-mount the directory into which the created packages are to be placed ("/tmp/repo" in the example above) from the host filesystem into the chroot, as otherwise the freshly built packages would be deleted again when pbuilder removes the throwaway chroot. This can be achieved by either passing a "--bindmounts" parameter to pbuilder as above or by adding a BINDMOUNTS entry to pbuilderrc.

Rebootstrap supports building gcc-7-based and gcc-8-based cross-toolchains; just set the "GCC_VER" parameter accordingly. After the build process has finished, you can find a repository with a cross-toolchain and a number of cross-built packages in /tmp/repo. Don't worry when rebootstrap stops the build process with an error - that is expected as rebootstrap tries to build further packages after the toolchain is ready and some of those don't yet properly cross-build for riscv64.

Please note that you have to delete the repository directory ("/tmp/repo" in the example above) if you want to re-run rebootstrap as rebootstrap currently doesn't properly handle a pre-filled repository directory.

Qemu

Starting from 2018-03-09, upstream qemu git contains RISC-V support; since 2018-04-12 RISC-V support is available in the qemu packages in unstable.

In system-emulation mode, qemu implements a "virt" board that allows running upstream kernels with virtio block and network devices and a serial console, and a "spike"-compatible board. For user-mode emulation, the the Linux kernel provides a very useful "binfmt-misc" feature that allows to transparently run foreign-architecture user-mode binaries with qemu.

Package repository

The Debian-Ports repository is the main package repository for the Debian riscv64 port - unless there are special circumstances, this is the repository that should be used as the base for further work on the port. A basic set of riscv64 packages has been imported into Debian-Ports on the weekend of 2018-03-24/25 and there are autobuilders running to keep the repository up-to-date.

For information about previously used historic package repositories please refer to the Attic page.

Creating a riscv64 chroot

The "standard" way of creating a chroot in Debian is by running debootstrap. Unfortunately debootstrap has one serious limitation, and that is that it can only work with a single suite (e.g. unstable). For riscv64 this has posed a problem for quite some time because there were a number of RISC-V-specific patches for packages that were only available in the "unreleased" suite, but not in the "unstable" suite. Therefore for a long time debootstrap couldn't create a fully-working riscv64 chroot of the type "standard", although creating chroots of the types "minbase" and "buildd" worked, but since 2019-05-29 everything that is required for creating a chroot of type "standard" with debootstrap is now available in unstable.

An alternative to debootstrap is mmdebstrap, which can create a chroot from packages distributed over multiple suites while providing a largely debootstrap-compatible user interface. Unless there are specific reasons for using debootstrap, the use of mmdebstrap is therefore recommended.

mmdebstrap

To create a riscv64 chroot in /tmp/riscv64-chroot with mmdebstrap, perform the following steps:

This uses qemu's "usernet" implementation which provides the virtual machine with NATed network access to the outside and works without root privileges, but it is rather slow and doesn't support all network protocols (ICMP support is limited). The "hostfwd" parameter sets up a port forwarding from port 22222 on the host to port 22 inside the virtual machine so that it is possible to log into the VM from the host.

With this file unpacked one can easily create an image to use with Qemu or similar, but still an external kernel is needed.

Status Log

2019-08-06

A new kernel package (linux-image-5.2.0-1-riscv64) that includes a backport of the new RISC-V kernel image header from kernel 5.3 has entered unstable. Together with OpenSBI (in unstable) and u-boot-qemu (in experimental) this allows using syslinux-style menus in U-Boot and makes setting up a virtual riscv64 system a lot easier.

2019-05-29

With version 0.176-1.1 elfutils has moved from unreleased to unstable which makes it now possible to use debootstrap to create a riscv64 chroot of type "standard".

U-Boot version 2019.07~rc1+dfsg-3 includes u-boot-sifive and u-boot-qemu package, which ship the sifive_fu540, qemu-riscv64, and qemu-riscv64_smode targets. The qemu-riscv64_smode target has been tested to work with qemu+opensbi to boot linux.

2019-01-05

Starting with version 4.19.13-1, the regular Linux kernel packages in unstable have riscv64 support and provide a kernel image that can be booted in qemu. There has also been progress on the issue of debootstrapping a riscv64 system - while creating a "standard" chroot with debootstrap still fails due to an upstream issue in elfutils, it is now possible to use debootstrap for creating chroots in the "minbase" and "buildd" variants. As a result, sbuild now works out-of-the-box on riscv64.

2018-12-11

Starting with version 3.2.1-9, libffi6 contains a backport of the riscv64 support from the upstream libffi7 development repository. The Debian riscv64 port had originally used a libffi7 snapshot instead of libffi6 because libffi6 didn't have any RISC-V support at that time and libffi upstream had originally planned to release libffi7 in May 2018. Based on this release schedule, all Debian architectures would have moved to libffi7 before the Buster freeze. In the meantime the upstream libffi7 release has been deferred for an unknown amount of time and riscv64 support for libffi6 has become available. Therefore the Debian riscv64 port moves from using libffi7 to using libffi6 to bring it in line with the other Debian ports. Due to ABI differences between libffi6 and libffi7, the transition requires re-building all packages that link against libffi, which will cause some packages to be temporarily uninstallable.

2018-09-13

A fix for the broken initrd handling in the Linux kernel has been committed to the upstream kernel git repository and will be part of the 4.19rc4 release. First versions of a patchset to support the qemu RISC-V "virt" board in u-boot have been posted to the upstream u-boot development list.

2018-08-21

The last driver bits required for booting the mainline kernel to userland on a qemu "virt" machine have been merged during the Linux 4.19 merge window. It is now possible to build a working kernel directly from upstream git without any additional patches. Please note that this currently only works for a "static" kernel, i.e. without initrd. Initrd support requires an additional patch that is planned to go upstream later in the 4.19 development cycle.

2018-08-04

Debian 9.5 has been released on 2018-07-14 and dak now accepts packages with riscv64 in their control file. As a result, a number of essential packages have been moved from the "unreleased" to the "unstable" suite and it is now possible to use debootstrap to create a "minbase" riscv64 chroot.

2018-06-29

The dpkg/stable update that is necessary to make dak accept packages which explicitly list riscv64 in their control file has been accepted into stable-proposed-updates and will be part of the upcoming 9.5 stable release. Once the 9.5 release is out and the system running dak has been updated accordingly, it will be possible to move essential packages like linux and glibc from unreleased to unstable and thereby enable the use of debootstrap for riscv64.

The dpkg 1.18.25 update for stable that would (among other things) have made the riscv64 architecture known to dak - and thereby have allowed uploads of packages that mention riscv64 in their control file to the archive - has been rejected by the stable release managers. The rejection hasn't been because of the riscv64 support but because of other factors, but it means it will unfortunately still take some time before we will be able to upload a number of core packages (e.g. linux and glibc) with riscv64 support enabled to the main archive.

2018-02-27

A pull request to include RISC-V support into upstream qemu has been sent.

Glibc 2.27 has been released with support for RV64. Support for RV32 hasn't been fully ready in time for the 2.27 release and will be added later on. A Debian package of glibc 2.27 has been uploaded to experimental.

Since version 1.19.0.5 dpkg includes support for the riscv64 architecture. Uploading of packages that reference riscv64 in their control file to the archive isn't yet possible though, as the Debian archive management software runs on Debian/stable and a corresponding stable update is still pending.

Version 4 of the qemu upstreaming patchset has been posted to the qemu-devel mailinglist.

2018-01-29

Support in glibc has been accepted and committed in the master branch, the release of glibc 2.27 should happen in the next few days.

Linux 4.15 was released a few days ago as well, with support for the userspace ABI needed by glibc. Drivers for this arch will be left for future releases, but ABI was the most important part.

2018-01-02

A first version of the qemu upstreaming patchset has been posted to the qemu-devel mailinglist.

2017-12-22

A new version of the glibc upstreaming patchset which matches the kernel code in Linux 4.15rc3 has been posted to the libc-alpha list.

2017-11-15

The pull request for the kernel has been accepted and the architecture-core patchset has been merged into the upstream kernel repository.

2017-11-13

A pull request for inclusion of the RISC-V architecture-core patchset into kernel 4.15 has been sent to Linus Torvalds.

Version 9 of the kernel upstreaming patchset has been posted to LKML on 2017-09-26. As planned after v8, it has been split into an architecture-core and a driver patchset. The RISC-V architecture maintainer has a kernel.org account now, which is a prerequisite for getting the patches into linux-next, but the actual inclusion into linux-next is still pending as the linux-next maintainer has announced that updating the linux-next tree will be on hold during the whole of October 2017.

2017-09-17

The kernel upstreaming patchset hasn't made it in into the kernel 4.14 merge window, so it now targets kernel 4.15. Version 8 of the patchset has been posted to LKML recently (note: the archive of the corresponding thread on lkml.org appears to be incomplete). While the patchset has received an overall positive review from kernel developer Arnd Bergmann, he and two other kernel developers have pointed out a few minor points that require some further discussion and probably some restructuring of the timer code. The plan for version 9 of the patchset is to address those issues and split the patchset into an architecture-core and a driver patchset. The architecture-core patchset can then hopefully be soon included in linux-next as a preparation for getting it merged during the kernel 4.15 merge window.

2017-07-30

The RISC-V upstream kernel patchset has gone through a number of review cycles, but hasn't made it into the kernel 4.13 merge window. Judging from the review comments, chances for an inclusion into kernel 4.14 look quite good, though. There are a number of open questions concering the RISC-V memory model, whose formal specification is still work-in-progress. The corresponding RISC-V foundation working group has announced that the formal memory model specification should be published in the near future (before end of 2017).

The upstream glibc maintainers have made clear that they require the kernel port to be accepted (at least as part of linux-next, preferably as part of a Linux release candidate) before the glibc support can be accepted for upstream inclusion. As a result, the upcoming glibc 2.26 release won't have RISC-V support. The earliest upstream glibc version that could have RISC-V support will therefore be 2.27, which is planned to be released around 02/2018.

2017-06-14

The first version of an upstreaming patchset for glibc has been posted to the upstream glibc development list (libc-alpha).

2017-05-22

The first version of an upstreaming patchset for the Linux kernel has been posted to the upstream Linux kernel mailinglist.

Upstream binutils 2.28 have been released with RISC-V support on 2017-03-02.

2017-02-06

The GCC support for RISC-V has been committed to the upstream GCC repository and will be part of the GCC 7 release. Commit list: 123456

2017-01-18

The binutils support for RISC-V has been accepted upstream in November/December 2016 and will be part of binutils 2.28 (expected to be released in Q1/2017).

The GCC support for RISC-V has been accepted for upstream inclusion by the GCC Steering Committee but is still pending the final stages of the technical review as there have been a number of review comments that need to be addressed in a new version of the upstreaming patchset. There is reason for hoping that the RISC-V support could make it into the GCC 7 release, but this depends on how fast the review process can be finished.

2016-02-19

The preparations for this port started in private a while ago, but nothing has been made public so far and nothing useful yet for users and developers.

The main reason is the lack of official support for this architecture in fundamental pieces of the toolchain (binutils, gcc, glibc), the main OS kernel (linux) or even other software that might help with the port (e.g. qemu). All of the mentioned pieces have support in progress and are considered to submit for upstreaming, but nothing definitive has happened at the moment.

In particular, a recent message informed about some upcoming changes to the supervisor specifications (the ABI), which will affect binutils at least. Starting a Debian port without the ISA being settled is not very good, since the effort will need to be restarted from scratch.

It is expected that this situation will change soon (within few months) and that progress on this port can be resumed.